A primary power supply design goal is the maintenance of a specified output voltage, within an allowable tolerance. An obstacle to achieving this goal is posed by the series resistance contained in the path leading to the output terminal. This resistance consists of parasitic resistance in the transformer, in addition to any other resistance inherent in the output path.
Because of this series resistance, the voltage at an output terminal varies with the load on that output terminal. This occurs in spite of the fact that the voltage of the primary transformer winding remains constant. As the load on an output terminal increases, additional current is drawn through its output path. The additional current causes an increased voltage drop across the series resistance, resulting in a reduction of the output terminal voltage. Conversely, a reduction of the load on an output causes the output terminal voltage to increase.
Secondary voltage control is often used to control the voltage of a power supply output terminal. Secondary voltage control operates by adjusting the voltage generated in the secondary winding of the transformer in response to the voltage sensed at the output terminal.
The effectiveness of secondary voltage control is limited, however, where more than one output terminal shares a common transformer. While the voltage of one output terminal is sensed and regulated by the secondary voltage control, the voltage at other output terminals fluctuate due to changes in their respective loads. The voltages of these other output terminals also fluctuate as the secondary voltage control adjusts the transformer voltage to regulate the voltage of the sensed output terminal.
Magnetic amplifier control is sometimes used to regulate the voltage of these output terminals which are not regulated by the secondary voltage control. This technique uses a magnetic amplifier in series with the output path, and a controllable current source capable of injecting current into the magnetic amplifier. When activated, this current source establishes a flux in the magnetic amplifier that is in opposition to the output terminal current. Regulation of the output terminal voltage is achieved by controlling the current source in response to the output terminal voltage.
Shunt regulators are also used to regulate output terminal voltages. Shunt regulators limit the increase in the output terminal voltage that normally occurs in response to load reduction. When the voltage of the output terminal exceeds a predetermined threshold, the shunt regulator diverts current from the output path, through a shunt resistor, to ground. This current diversion increases the load on the output, thereby increasing the voltage drop across the output path series resistance. As a result, the output terminal voltage is lowered. The resulting upper limit on output terminal voltage is obtained at the expense of power dissipated in the shunt resistor.
In view of the foregoing, it is desirable to have an inexpensive device to regulate the output terminal voltages for a multiple output power supply. It is also desirable to achieve this regulation while minimizing the amount of power dissipated. Further, it is desirable that the device be able to operate in conjunction with, or in the absence of, other voltage control circuits, such as secondary voltage control or the like.